What the numeral actually settles
Three dimensions and an end-cap pattern: outside diameter, height, and the shape of the holes at each end. That is enough to answer the only question the numeral exists to answer — will this element drop into that housing and seal.
It fixes them well. Cartridge types are among the more reliable compatibility conventions on this shelf, precisely because two large manufacturers established them and the third-party trade has every commercial reason to match them exactly.
What a type number cannot tell you
It does not promise a filtration fineness. Nothing in the type says what size of particle the medium catches, and two elements of the same type can differ several-fold in the paper they are pleated from.
It does not promise a filter area. A type is a envelope; how much pleated paper a maker fits inside it is a design choice, and the difference shows up directly in how often the cartridge needs rinsing.
And it does not promise a service life. A cartridge lasts as long as its pleats reopen after rinsing, which depends on the water, the pump, and whether it was ever allowed to dry between runs.
The figures that outrank the numeral
Filter area, in square metres, where it is published — the figure that actually predicts the interval between cleanings. Pleat count, which is a proxy for area when area is absent. Medium, because polyester and paper behave differently when wet and when dried. And pack economics: a cartridge is bought several times a season, so the price of a four-pack matters more to a summer than the price of the housing did.
Where a type decides the purchase
Filter cartridges, obviously, where the type is the first line on every page. Cartridge filter systems, where the type a housing takes is a lifetime commitment made at purchase and is therefore printed above the flow rate. And outdoor spas, where the same convention runs under different names — the small S-series elements most inflatable spas use are a family of their own, and a spa with a discontinued element is a spa with a component life.
What readers keep writing in to ask
Is a higher number a better cartridge?
No. The numerals are names, not grades. Type VI is physically larger than type I, which means more filter area and a longer interval between cleanings, but that is a consequence of size rather than of quality. A large cartridge of coarse paper filters worse than a small one of fine.
Can I put a type II cartridge in a type I housing?
Only if the maker says so. The types differ in diameter and height, and a cartridge that does not seal against both end caps lets water past it rather than through it — which produces a filter that appears to be working and is not. Compatibility is a published list, never a judgement made by eye.
Why does the same cartridge have three different names?
Because each maker numbers its own range and the third-party manufacturers cross-reference them. A single element can be a type A in one catalogue, a 29000 in another and a PLF-something in a third. Pages here print the type together with the references a maker lists as equivalent.
How is filter area published?
In square metres or square feet, and often not at all. Where it is published, it is the more informative figure than the type — it is what decides how long a cartridge runs between cleanings. Where it is not, the page records that it was not published and compares pleat count instead if that is given.
Last reviewed 21 September 2026